Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers in particular convert electrical signal to acoustic signals (sound waves) in a transmit mode (e.g., a speaker application), and/or convert received acoustic waves to electrical signals in a receive mode (e.g., a microphone application). The functional relationship between the electrical and acoustic signals of an acoustic transducer depends, in part, on the transducer's operating parameters, such as natural or resonant frequency, acoustic receive sensitivity, acoustic transmit output power and the like.
Various types of transducers include micro-electromechanical systems (MEMS) transducers, such as piezoelectric micro-machined ultrasonic transducers (PMUTs), which are especially sensitive to operating environments. In order to provide some protection, a transducer may be passivated on top and bottom surfaces of the active transducer, in accordance with a simple passivation scheme.
For example, FIGS. 1A and 1B are cross-sectional diagrams illustrating a conventional transducer device.
Referring to FIG. 1A, transducer device 100 includes a substrate 110 with an opening 115, over which various layers are stacked to form an active transducer 150. More particularly, active layers of the transducer device 100 are formed between a first passivation layer 121 and a second passivation layer 122, which are stacked on the substrate 110. The active layers include a first conductive layer 131 formed on the first passivation layer 121, a piezoelectric layer 140 formed on the first conductive layer 131 and a second conductive layer 132 formed on the piezoelectric layer 140. The first and second conductive layers 131 and 132 act as first and second electrodes for the active transducer 150.
The first and second passivation layers 121 and 122 partially protect the active layers from moisture, corrosives, contaminants, debris and the like, but only with respect to the bottom (at opening 115) and top surfaces, respectively. In other words, the side surfaces or edges of the first and second conductive layers 131 and 132 and the piezoelectric layer 140 are exposed, having no passivation layer protection, e.g., after separation or dicing of the transducer device 100 from a wafer.
Accordingly, degradation due to exposure to the environment may occur in the active layers, for example, by channeling along the active layers through the exposed edges, as indicated by representative degraded portion 144 of the piezoelectric layer 140 in FIG. 1B. Such degradation compromises the integrity of the active layers, eventually causing the transducer device 100 to operate less efficiently and/or to malfunction, and otherwise shortening its operational life, particularly in harsh environments.